PALEOENVIRONMENTAL IMPLICATIONS OF TIME-AVERAGING AND TAPHONOMIC VARIATION OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA

Palaios ◽  
2020 ◽  
Vol 35 (2) ◽  
pp. 49-66 ◽  
Author(s):  
EMILY K. RYAN ◽  
MICHAEL J. SOREGHAN ◽  
MICHAEL M. MCGLUE ◽  
JONATHAN A. TODD ◽  
ELLINOR MICHEL ◽  
...  
Keyword(s):  
Water Depth ◽  
Lake Tanganyika ◽  
Age Distribution ◽  
River Mouth ◽  
Ice Age ◽  
Time Averaging ◽  
Biological Processes ◽  
Production Rates ◽  
Depositional Processes ◽  
Shell Beds

ABSTRACT The lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ∼ 100–650 BP). In addition, age-distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record.

scholarly journals Tidal variability of nutrients in a coastal coral reef system influenced by groundwater

2018 ◽  
Vol 15 (4) ◽  
pp. 997-1009 ◽  
Author(s):  
Guizhi Wang ◽  
Shuling Wang ◽  
Zhangyong Wang ◽  
Wenping Jing ◽  
Yi Xu ◽  
...  
Keyword(s):  
Coral Reef ◽  
Water Column ◽  
Water Depth ◽  
Neap Tide ◽  
Groundwater Discharge ◽  
Spring Tide ◽  
Biological Processes ◽  
Reef System ◽  
Nitrite Nitrate ◽  
Coral Reef System

Abstract. To investigate variation in nitrite, nitrate, phosphate, and silicate in a spring–neap tide in a coral reef system influenced by groundwater discharge, we carried out a time-series observation of these nutrients and 228Ra, a tracer of groundwater discharge, in the Luhuitou fringing reef at Sanya Bay in the South China Sea. The maximum 228Ra, 45.3 dpm 100 L−1, appeared at low tide and the minimum, 14.0 dpm 100 L−1, appeared during a flood tide in the spring tide. The activity of 228Ra was significantly correlated with water depth and salinity in the spring–neap tide, reflecting the tidal-pumping feature of groundwater discharge. Concentrations of all nutrients exhibited strong diurnal variation, with a maximum in the amplitude of the diel change for nitrite, nitrate, phosphate, and silicate in the spring tide of 0.46, 1.54, 0.12, and 2.68 µM, respectively. Nitrate and phosphate were negatively correlated with water depth during the spring tide but showed no correlation during the neap tide. Nitrite was positively correlated with water depth in the spring and neap tide due to mixing of nitrite-depleted groundwater and nitrite-rich offshore seawater. They were also significantly correlated with salinity (R2  ≥  0.9 and P < 0.05) at the ebb flow of the spring tide, negative for nitrate and phosphate and positive for nitrite, indicating the mixing of nitrite-depleted, nitrate- and phosphate-rich less saline groundwater and nitrite-rich, nitrate- and phosphate-depleted saline offshore seawater. We quantified variation in oxidized nitrogen (NOx) and phosphate contributed by biological processes based on deviations from mixing lines of these nutrients. During both the spring and neap tide biologically contributed NOx and phosphate were significantly correlated with regression slopes of 4.60 (R2  =  0.16) in the spring tide and 13.4 (R2  =  0.75) in the neap tide, similar to the composition of these nutrients in the water column, 5.43 (R2  =  0.27) and 14.2 (R2  =  0.76), respectively. This similarity indicates that the composition of nutrients in the water column of the reef system was closely related with biological processes during both tidal periods, but the biological influence appeared to be less dominant, as inferred from the less significant correlations (R2  =  0.16) during the spring tide when groundwater discharge was more prominent. Thus, the variability of nutrients in the coral reef system was regulated mainly by biological uptake and release in a spring–neap tide and impacted by mixing of tidally driven groundwater and offshore seawater during spring tide.

Paleocommunity information retrieval vs. shell accumulation mode in Paleozoic carbonates: examples from the Lebanon Limestone (Middle Ordovician), Tennessee, U.S.A.

1992 ◽  
Vol 6 ◽  
pp. 81-81
Author(s):  
David J. Davies ◽  
Molly F. Miller
Keyword(s):  
Information Retrieval ◽  
High Energy ◽  
Sediment Surface ◽  
Time Averaging ◽  
Short Term ◽  
Diagenetic Alteration ◽  
Shell Beds ◽  
Carbonate Shell ◽  
Shell Bed

Compared to their terrigenous counterparts, carbonate shell accumulations have until recently been relatively little studied to determine either descriptive or genetic classifications of shell bed types, the preservation potential of each type, or their relative ability to preserve community-level information. A partial classification of Paleozoic carbonate shell-rich soft sediment accumulations is proposed using sedimentation patterns in the Lebanon limestone of the Stones River Group. Paleoecological information preserved therein is then contrasted by shell bed type. The Lebanon represents typical Ordovician shallow to moderate subtidal carbonate shelf deposits in outcrops flanking the Nashville Dome and peritidal deposits in the Sequatchie Anticline of Eastern Tennessee; shell beds alternate with shell poor sediments (micrites, wackestones and diagenetically enhanced dolomites and clay-rich partings).None of the analyzed shell beds was strictly biological in origin; most are sedimentological although >10% are combined sedimentological/diagenetic. While the majority are single simple shell beds, >20% are amalgamated. All are thin (1 shell to 15 cm) stringers that pinch and swell showing poor lateral continuity (outcrop scale, tens to hundreds of meters) likely enhanced by burial dissolution. These shell beds differ greatly in fabric (packing/sorting), clast composition, taphonomic signature, and intensity of time averaging; thus community information retrieval is biased in predictable patterns. Virtually no shell beds show common shell dissolution or encrustation from long-term sediment surface exposure or hardground formation. Five major categories of accumulation are herein proposed using a DESCRIPTIVE, non-genetic terminology modified from previous works of DJD, as well as a Genetic interpretation for each. These are easily distinguished in the field and are also discriminated by Q-mode cluster analysis.Categories include, in decreasing frequency of occurrence: 1. SHELL GRAVELS; Storm/“event” beds: Sharp bases; poorly sorted coarse basal bioclasts and/or intraclasts, often with no preferred orientation; clasts fine upward to comminuted shell material and micrite. Horizontal platy brachiopods often cap the beds. High diversity and a wide range in shell alteration is represented, from whole unaltered brachiopods to minor abraded fragments, indicating extreme time averaging and poor resolution of short-term community dynamics. 2. COMMINUTED SHELLY LS; Current/ripple concentrations: Small tidal channel fill and discrete ripple trough accumulations are composed of cross-stratified bioclastic deposits with local concentrations of rip-ups. Beds are not graded; typically clasts are abraded, rounded and concordant with cross-beds. Intense time averaging and mixing of discrete communities is inferred due to continual reworking in these background deposits. 3. SHELL/CEMENT LS; Early cementation beds: Intense early diagenetic alteration is inferred due to red discoloration and rapid intergranular cementation; some beds show diagenetic micritic rinds. Beds may be brecciated and show deep burial stylolitization cutting bioclasts and cement. They may represent zones of preferred early cementation rather than a change in shell accumulation rate. Many shells from some beds show little postmortem alteration; these units may preserve much of the original community structure. 4. DENSE SHELL PAVEMENTS; Subtidal surficial pavements: Single layers of shells, commonly concave down, overlie mudstones/wackestones with no basal erosion. No obrution deposits were noted. Bioclasts are typically disarticulated and reoriented, but are not substantially abraded, broken, or dissolved. Diversity is low. Only minor temporal and lateral community mixing with small environmental fluctuation is indicated. 5. VERTICALLY IMBRICATE SHELLY LS; High energy beach zones: Platy whole and major fragments of brachiopods are deposited in low diversity, high angle imbricate beds. Less postmortem reworking and time averaging is evident compared to types 1 and 2.Thus, the most common (physically reworked) shell bed types show the most intense loss of short-term paleocommunity information. There are surprisingly few insitu community pavements or obligate long-term accumulations. This pattern differs from some described Ordovician carbonates, which may contain common community beds or hardgrounds/hiatal accumulations. This implies a relatively low rate of net sediment accumulation on a shallow, periodically wave swept shelf, and no major flooding surfaces or other indications of significant sea level change. Delineation of the sequence stratigraphic position of these carbonates is enhanced from this type of integrated community/biostratinomic analysis.

Time and taphonomy: Actualistic evidence for time-averaging of benthic foraminiferal assemblages

1993 ◽  
Vol 6 ◽  
pp. 34-56 ◽  
Author(s):  
Ronald E. Martin
Keyword(s):  
High Resolution ◽  
Ecosystem Management ◽  
Deep Sea ◽  
Time Averaging ◽  
Biological Processes ◽  
Exploration And Exploitation ◽  
Short Term ◽  
Resource Exploration ◽  
To Come ◽  
Paleoclimate Modelling

For more than half a century, microfossils–especially foraminifera–have been widely used as stratigraphic markers and paleoenvironmental indicators. Although increasing emphasis has been placed on their use in high-resolution paleoclimate studies, the time-scales involved in most microfossil-based stratigraphic investigations have remained relatively coarse (hundreds-of-thousands to millions of years). My intent herein is to try to come to grips with the interplay between time-averaging of benthic foraminiferal assemblages and stratigraphic resolution, and the implications for recognition of short-term physical and biological processes. These sorts of considerations deserve much closer scrutiny as the applied Earth sciences continue to move from a base of resource exploration and exploitation to one of paleoclimate modelling and ecosystem management (Martin, 1991; Corliss, 1993). The potential stratigraphic and paleoenvironmental resolution of foraminiferal assemblages is assessed using concepts derived from the age analysis of deep-sea assemblages.

Response of benthic species to post-glacial sea-level rise on the northern Adriatic shelf revealed by stratigraphic unmixing of fossil assemblages

2020 ◽  
Author(s):  
Rafał Nawrot ◽  
Daniele Scarponi ◽  
Adam Tomašových ◽  
Michał Kowalewski
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Age Distribution ◽  
Ecosystem Dynamics ◽  
Time Averaging ◽  
Burial Depth ◽  
Shelf Area ◽  
Benthic Species ◽  
Northern Adriatic ◽  
Fossil Assemblages

&lt;p&gt;Late Quaternary fossil record offers a window into ecosystem dynamics during episodes of abrupt climate warming and sea-level rise following the Last Glacial Maximum, but in marine settings ecological inferences might be hindered by high time-averaging affecting transgressive deposits. However, the signature of temporal shifts in local skeletal production rates may be preserved in the age-frequency distributions (AFDs) of death assemblages. We use carbonate-target radiocarbon ages of 191 shells to examined variation in AFDs among four bivalves species collected from a 2.3-meter-long core recording the post-glacial transgression on the northern Adriatic shelf over the last the last ~14,500 yr.&lt;/p&gt;&lt;p&gt;The scale of time-averaging within species (interquartile age range) varied from 200 to 7,400 yrs, while the between-species age offsets (differences between the median ages of species) ranged from ~2 to 6,400 yrs within 5-cm-thick core intervals. Although the median ages of &lt;em&gt;Varicorbula&lt;/em&gt;, &lt;em&gt;Timoclea&lt;/em&gt; and &lt;em&gt;Parvicardium&lt;/em&gt; increased with increasing burial depth, shells of &lt;em&gt;Lentidium&lt;/em&gt; appeared age-homogeneous throughout the core. Age unmixing revealed a single massive peaks in the abundance of this opportunistic, shoreface species around 14 cal ka BP, coincident with the initial marine flooding of this shelf area during the melt-water pulse 1A. Moreover, a prominent gap in the AFDs between 11 and 12.5 cal ka BP corresponds to a minor sea-level fall associated with the Younger Dryas cold spell. Importantly, the reconstructed onsets and durations of shell production pulses across the four species are consistent with independently-derived relative sea-level history at the site. The species gradually replaced each other through time as the dominant component of the assemblage in accordance with their bathymetric preferences estimated from surveys of the modern Adriatic benthic fauna.&lt;/p&gt;&lt;p&gt;The diachronous production histories of four bivalve species coupled with subsequent exhumation of old shells and burial of younger shells through bioturbation and sediment reworking resulted in the ecologically mixed fossil assemblages. These assemblages are thus characterized by multi-modal age distribution and millennial-scale age offsets between species co-occurring in the same stratigraphic increments. Although this stratigraphic homogenization and disorder greatly limits the resolution of the raw stratigraphic record, our results demonstrate the power of AFDs to capture shifts in abundance of benthic species during recent episodes of rapid sea-level rise. Fossil assemblages from transgressive deposits preserved on continental shelves represent a rich and underutilized source of data on long-term biotic responses to global climate change and associated shifts in sea level.&lt;/p&gt;

scholarly journals Age and growth of Palaeoloxodon huaihoensis from Penghu Channel, Taiwan: significance of their age distribution based on fossils

PeerJ ◽  
2021 ◽  
Vol 9 ◽  
pp. e11236
Author(s):  
Jia-Cih Kang ◽  
Chien-Hsiang Lin ◽  
Chun-Hsiang Chang
Keyword(s):  
Age Distribution ◽  
Age Groups ◽  
Age Determination ◽  
Dental Material ◽  
Age And Growth ◽  
Ice Age ◽  
Older Individuals ◽  
African Forest Elephant ◽  
Tooth Width ◽  
Size And Morphology

Dental material attributed to Palaeoloxodon huaihoensis from the Middle to Late Pleistocene were recovered over decades from the Penghu Channel during commercial fisheries activities. The National Museum of Nature Science (NMNS) has a collection of such dental material, which differs in size and morphology and likely represents ontogenetic variation and growth trajectory of various age groups of P. huaihoensis. However, little is known regarding age determination. By using length of dental material, enamel thickness (ET), and plate counts, we established the method to distinguish the age of the species, which is directly derived from the extant African forest elephant Loxodonta africana. When measuring signs of allometric growth, we found that in both the upper and lower jaws, tooth width was correlated negatively with lamellar frequency but positively with ET. In the same age group, the number of lamellae was higher in P. huaihoensis than in L. africana. The reconstructed age distribution indicated no difference in the upper or lower jaw. Notably, within our sample, P. huaihoensis is skewed towards adult and older individuals with median age between 33–34.5 years and differed significantly from that of Mammuthus primigenius in the European Kraków Spadzista site. This age distribution pattern is speculated to be related to the harsh environmental conditions and intense intraspecific competition among P. huaihoensis during the last ice age.

Hydrophysical parameters of seawater in front of Ustka during the summer season 2013

2017 ◽  
Vol 32 (1) ◽  
pp. 37-46
Author(s):  
Maciej Kałas ◽  
Ewelina Misiewicz
Keyword(s):  
Cross Section ◽  
Water Depth ◽  
Coastal Waters ◽  
Water Distribution ◽  
River Mouth ◽  
Water Density ◽  
Local Wind ◽  
Entire Cross Section ◽  
Directional Distributions

The article presents spatial distribution of hydrophysical parameters of coastal waters near Ustka, stretching towards the eastern part of Słupsk Bank which appeared there in July 2013. It covers studies of distribution of temperature, salinity and seawater density across the entire cross-section of the water depth. Additionally, directional distributions of the water flows recorded at Baltic Sea, as well as information on local wind speed and direction derived from numerical model of the atmosphere are presented. The resulting image of water density distribution across the entire cross-section of the water depth under steady conditions of stratification allowed determination of character and extent of river water distribution seawards of the Słupia River mouth during summer 2013

Sign in / Sign up

Export Citation Format

Share Document